By 8 weeks of gestational age, the Leydig cells of the developing testis
are capable of producing testosterone under the stimulation of human chorionic
gonadotropin. Circulating testosterone causes development of tissues with
testosterone receptors. Normally, testosterone is taken into the cell and
bound by the androgen receptor. In the cytoplasm, it is converted to dihydrotestosterone
by the enzyme 5-alpha-reductase. Dihydrotestosterone is at least 4 times
more potent than testosterone.

The genital eminence, an external mound arising between the umbilicus
and the tail, is made up of the genital tubercle and the genital swellings.
The urogenital sinus opens at the base of the genital tubercle, between
the genital swellings. These structures form identically in male and female
embryos up to 7 weeks gestational age.

At 9 weeks of gestational age, and under the influence of testosterone,
the genital tubercle starts to lengthen. In addition, the genital swellings
(also called the labio-scrotal folds) enlarge and rotate posteriorly. As
they meet, they begin to fuse from posterior to anterior. As the genital
tubercle becomes longer, two sets of tissue folds develop on its ventral
surface on either side of a developing trough, the urethral groove. The
more medial endodermal folds will fuse in the ventral midline to form the
male urethra. The more lateral ectodermal folds will fuse over the developing
urethra to form the penile shaft skin and the prepuce. As these two layers
fuse from posterior to anterior, they leave behind a skin line: the median
raphe.

By 13 weeks, the urethra is almost complete. A ring of ectoderm forms
just proximal to the developing glans penis. This skin advances over the
corona glandis and eventually covers the glans entirely as the prepuce
or foreskin.

Clicking on the hypertext link that follows will show you a movie made
by combining photographs of male fetuses at various stages of penile development.
These images are from Glennister, TW: The origin and fate of the urethral
plate in man. Journal of Anatomy 288:413, 1954. I am indebted
to Cambridge University Press for granting me the right to use them in
this tutorial. Feel free to pause the movie and run it backwards and forwards
in order to read the text superimposed over the images. See
embryology movie.